The present embodiments relate to generating motion information for an at least partially moving examination region.
When gradients are used for spatial encoding when acquiring magnetic resonance data sets, the signals picked up are affected by motion. This is true for both simple imaging and spectroscopic data sets with spatial resolution.
Motion artifacts may be produced in subjects by cardiac motion and also by respiratory motion. In order to avoid artifacts due to motion of the heart, it is known to evaluate an ECG signal of the subject or patient and trigger the data acquisition as a function of the ECG signal. This provides that the data sets are acquired in the same phase of the cardiac cycle.
A number of methods for determining respiratory motion are known. The motion of the abdomen may be determined by a measuring belt. This does not influence MR measurements but only supplies approximate information about the deformation of the abdomen in the examination region.
It is therefore also known to pick up navigator echoes. One or more 2D slices or 3D volumes with predetermined orientations are read, and the differences allow translations and rotations of the region in question to be inferred in otherwise identical acquisition conditions. This method allows motions to be detected very precisely, but such measurements also cause the signal to become saturated in the image region of an image. Also, the navigator measurements are to be acquired in measurement pauses of a measurement sequence. The navigator measurements therefore may not be used with steady state sequences such as FLASH or TrueFISP.
It is set out in Andreychenko et al., Noise variance of an RF receive array reflects respiratory motion: a novel respiratory motion predictor, Proc. Intl. Soc. Mag. Reson. Med., 22, page 92, 2014 how to pick up noise signal with a coil of a magnetic resonance system. The amplitude of this varies with the motion of the abdomen. The noise signal may also only be picked up in measurement pauses, and approximately 40,000 averages are used to obtain a usable SNR. The measurement then takes 40 ms even for samples with a duration of only 1 μs. This provides that the method may only be combined with steady state sequences in exceptional instances.
However, motion artifacts also result when other imaging modalities such as SPECT or PET, which average signals during different motion phases of the respiratory cycle, are used.